Wrong frequency metadata in BOE/4190

This is a bug reported on the forum: https://forum.db.nomics.world/t/wrong-metadata-in-a-series/164

In BOE/4190/dataset.json, the dimension freq is declared but isn't specified for each series.

Note that in the source, the data are monthly but the date used is the last day of the month. The writing of the period in the TSV should be corrected to a month and not a day.

We should also check what the API is doing when freq is declared but doesn't have a value for a given series.